The present invention relates to a fuel supply system for supplying a fuel from a fuel tank to an engine.
It is known that a fuel tank mounted on a vehicle is separately constructed of a main tank and a subtank provided in the main tank, so as to reliably supply a small amount of fuel in the fuel tank to the engine even when the fuel is deflected in the fuel tank upon inclination or acceleration of the vehicle. In the fuel tank of this type, the fuel in the main tank is once supplied to the subtank, and a predetermined amount of the fuel is stored in the subtank. The fuel in the subtank is supplied to the engine. Accordingly, even when the fuel in the main tank is deflected, the fuel in the subtank can be reliably supplied to the engine without idling of a pump.
Further, it is also known that an unconsumed part of the fuel supplied to the engine is returned to the subtank. In such a fuel supply system, when a constant amount of fuel is supplied from the subtank to the engine in spite of a large amount of return fuel from the engine to the subtank, the fuel is unnecessarily circulated between the subtank and the engine to cause not only a loss of energy but also an increase in temperature of the fuel in the subtank due to a hot return fuel from the engine. Such an increase in temperature of the fuel causes the generation of fuel vapor in the subtank, and the fuel vapor tends to induce vapor lock in the pump or the engine.
Further, when the level of the fuel in the subtank exceeds a predetermined level, the fuel overflows into the main tank. Accordingly, the fuel in the main tank is warmed by such an overflowing fuel from the subtank including a hot return fuel from the engine, thus causing the generation of fuel vapor in the main tank.
The above-mentioned fuel supply system of the type where an unconsumed part of the fuel supplied to the engine is returned to the subtank is known from Japanese Patent Laid-open Publication No. 56-107952 and Japanese Utility Model Laid-open Publication No. 62-56759, for example. In these references, it is proposed that an amount of return fuel from the engine to the fuel tank is detected, and a driving power for the pump is reduced with an increase in the amount of the return fuel to thereby make the return fuel amount constant.
This prior art technique can be applied to the subtank so as to suppress undue circulation between the subtank and the engine. However, the adaptation of this technique to the suppression of undue circulation between the subtank and the main tank is not considered in the above references. Accordingly, it is not possible to prevent that a part of the return fuel in the subtank is circulated from the subtank to the main tank to cause a temperature increase in the fuel in the main tank and accordingly generate the fuel vapor in the main tank.